Process for the preparation of block copolymers for tire tread compositions, and these copolymers

ABSTRACT

A process for the preparation of a functionalized, coupled or starred block copolymer which is usable in a sulphur-cross-linkable rubber composition of reduced hysteresis in the cross-linked state, one at least of said blocks consisting of a polyisoprene and at least one other block consisting of a diene elastomer other than a polyisoprene the molar ratio of units originating from one or more conjugated dienes of which is greater than 15%. The process includes the steps of: copolymerization of one or more monomers comprising a conjugated diene other than isoprene using a catalytic system comprising at least one hydrocarbon solvent, a compound A of a metal of group IIIA, a compound B of an alkaline-earth metal and a polymer initiator C comprising a C—Li bond which is formed of a monolithiated polyisoprene intended to form the polyisoprene block, and addition to the product of the copolymerization of a functionalizing, coupling or starring agent comprising an acetoxy group and of formula R n —Sn—(O—CO—R′) 4−n , where n is a natural integer of from 0 to 4 and where R and R′ are each alkyl, cycloalkyl, aryl or aralkyl groups which may be identical or different, so that said or each block formed of a diene elastomer other than a polyisoprene is functionalized, coupled or starred.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/EP2004/004722, filed May 4, 2004, published in French on Nov. 18,2004 as WO 2004/099277, and claims priority based on French ApplicationNo. 03/05589, filed May 7, 2003, the disclosures of both applicationsbeing incorporated herein in their entirety.

BACKGROUND

The present invention relates to a process for the preparation of afunctionalized, coupled or starred block copolymer which is usable in asulphur-cross-linkable rubber composition of reduced hysteresis in thecross-linked state, to such a block copolymer and to a rubbercomposition incorporating it which is usable in a tire tread. Theinvention also relates to this tread and to a tire incorporating ithaving reduced rolling resistance.

DESCRIPTION OF RELATED ART

The tire industry is constantly endeavoring to reduce the hysteresis ofmixes in order to limit fuel consumption and thus conserve theenvironment.

This reduction in hysteresis must be achieved while maintaining or evenimproving the processability of the mixes. Numerous approaches forreducing hysteresis have already been investigated. Chain endfunctionalization has appeared to be a particularly promising method.

Most of the proposed methods have involved seeking out adjacentfunctions on completion of polymerization which are capable ofinteracting with carbon black, for example contained in star polymers ortin-coupled polymers. European Patent Specification EP-A-709 235 may bementioned by way of example. Other functions which interact with carbonblack have also been attached to chain ends, such as4,4′-bis(diethylaminobenzophenone), which is also known as DEAB, orother amine functions. Patent specifications FR-A-2 526 030 and U.S.Pat. No. 4,848,511 may be mentioned by way of example.

For some years, it has been possible to use silica and research has beenunder way to find functions capable of interacting with this filler.Patent specification FR-A-2 740 778, which discloses for examplefunctions comprising a silanol group, may in particular be mentioned inthis connection. Mention may also be made of specification U.S. Pat. No.5,066,721, which discloses alkoxysilane or aryloxysilane functions, oralternatively specification U.S. Pat. No. 3,244,664.

Most of these solutions, whether for carbon black or for silica, bringabout a genuine limitation of hysteresis and an increased level ofreinforcement of the corresponding compositions. Unfortunately, it isalso generally the case that these improvements result in greaterdifficulty in processing these compositions.

Other means of reducing hysteresis which do not affect the processing ofthe mixes have thus been sought.

In particular, using polymers with a low hysteresis potential, inparticular polyisoprene, has appeared to be a promising approach.However, directly using this type of polymer does not always provide asatisfactory compromise between dynamic modulus and hysteresis.

In order to overcome this disadvantage, the attempt has been made to useblock copolymers comprising a polyisoprene block.

Block copolymers are generally composed of materials in segregatedphases. Diblock polyisoprene/polystyrene copolymers, the synthesis ofwhich has been comprehensively described in the literature, may bementioned by way of example. These diblock copolymers are known toexhibit valuable impact-resistance properties.

Block copolymers comprising polyisoprene and polybutadiene blocks(abbreviated to IR and BR respectively) have also been described in theliterature.

Certain post-polymerization reactions convert these elastomers intothermoplastic materials. For example, when hydrogenating a triblockBR/IR/BR copolymer, the butadiene fraction forms a crystallinepolyethylene, while the isoprene fraction gives rise to a rubberyethylene/butylene-type material.

Hydrochlorination of these materials may also impart crystallineproperties thereto.

Diblock IR/SBR copolymers (polyisoprene/copolymer of styrene andbutadiene) have been described in European Patent Specification EP-A-438967, in relation to a reinforcing filler specifically comprising carbonblack. The number-average molecular weight of the IR block is preferablybetween 70,000 and 150,000 g/mol, while that of the SBR block ispreferably between 220,000 and 240,000 g/mol. Furthermore, the ratio ofthe number-average molecular weight of the IR block to that of the SBRblock must be greater than 33% and may be as much as 300%.

The rubber compositions described in this document may be of a variablestructure, which is lamellar when said ratio is of the order of 33%, andspherical when said ratio is of the order of 300%.

However, for all these values of said ratio ranging from 33% to 300%, itshould be noted that the relatively high number-average molecular weightof the IR block always results in marked segregation of the phasescorresponding to the IR and SBR blocks respectively, owing to the highcontents of 1,4-linkages in the IR block.

IR/BR block copolymers have also been considered as compatibilizingagents for blends of polyisoprene and polybutadiene.

The paper by D. J. Zanzig, F. L. Magnus, W. L. Hsu, A. F. Halasa, M. E.Testa, Rubber Chemistry and Technology vol. 66, pp. 538-549 (1993),which mentions the use of IR/BR block copolymers containing 80% or 50%IR, may be mentioned in this connection. At these relative contents, thenumber-average molecular weight of the IR block is always equal to orgreater than 200,000 g/mol, and as a result the blocks of thesecopolymers also form segregated phases.

Mention may also be made in this connection of the paper by R. E. Cohen,A. R. Ramos, Macromolecules Vol. 12, No. 1, 131-134 (1979). In thispaper, the diblock copolymers used comprise IR blocks of anumber-average molecular weight of 104,000 g/mol, or of 133,000 g/mol.The relatively high molecular weight of the IR blocks and BR blocks alsoresults in great segregation of the phases associated with these twoblocks.

SUMMARY OF THE INVENTION

One aim of the present invention is to propose a process for thepreparation of a functionalized, coupled or starred block copolymerwhich is usable in a sulphur-cross-linkable rubber composition, such asa tire tread composition, and which imparts to this composition areduced hysteresis in the cross-linked state, this copolymer being suchthat one at least of its blocks is formed of a polyisoprene, at leastone other block consisting of a diene elastomer other than apolyisoprene, the molar ratio of units originating from one or moreconjugated dienes of which is greater than 15%. This object is achievedin that the Applicants have unexpectedly discovered that:

(i) the copolymerization of one or more monomers comprising at least oneconjugated diene other than isoprene using a catalytic system comprisinga hydrocarbon solvent, a compound A of a metal of group IIIA, a compoundB of an alkaline-earth metal and a polymer initiator C comprising a C—Libond which is formed of a monolithiated polyisoprene intended to formsaid polyisoprene block, and

(ii) the addition to the product of this copolymerization of afunctionalizing, coupling or starring agent comprising an acetoxy groupof formula (1) R_(n)—Sn—(O—CO—R′)_(4−n), where n is a natural integer offrom 0 to 4 and where R and R′ are each alkyl, cycloalkyl, aryl oraralkyl groups which may be identical or different, so that said blockformed of a diene elastomer other than a polyisoprene is functionalized,coupled or starred,

makes it possible to prepare a functionalized, coupled or starred blockcopolymer which is usable in a sulphur-cross-linkable rubber compositionfor a tire tread comprising carbon black as reinforcing filler owing tothe improved interaction of this copolymer with the carbon black, thiscopolymer according to the invention making it possible to optimizesignificantly for this composition, in the cross-linked state, theresults of the reduction in hysteresis and, in the non-cross-linkedstate, the results of processing ability.

DETAILED DESCRIPTION OF THE INVENTION

In particular, taking as reference the hystereses relating to “control”diene elastomers the molar ratio of units originating from conjugateddienes of which is greater than 15%, for example styrene/butadienecopolymers (SBR) or IR/SBR block copolymers both of which arenon-modified (i.e. neither functionalized, nor coupled, nor starred), ablock copolymer according to the invention is characterized by ahysteresis which is more reduced than that relating to these “control”elastomers.

A diene elastomer the molar ratio of units originating from conjugateddienes of which is greater than 15% which is capable of forming saidblock other than said polyisoprene block is understood to mean anyhomopolymer obtained by polymerization of a conjugated diene monomerother than isoprene having 4 to 12 carbon atoms, or any copolymerobtained by copolymerization of one or more dienes conjugated togetheror with one or more vinyl aromatic compounds having from 8 to 20 carbonatoms.

Suitable conjugated dienes are, in particular, 1,3-butadiene, 2,3-di(C1to C5 alkyl)-1,3-butadienes such as, for instance,2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene,2-methyl-3-ethyl-1,3-butadiene, 2-methyl-3-isopropyl-1,3-butadiene,phenyl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.

Suitable vinyl aromatic compounds are in particular styrene, ortho-,para- or meta-methylstyrene, the commercial mixture “vinyltoluene”,para-tert. butylstyrene, methoxystyrenes and vinylmesitylene.

Advantageously, butadiene and a vinyl aromatic compound, such asstyrene, are used as monomers to be copolymerized with the polyisoprenepresent in the lithiated initiator C, preferably for obtaining an IR andSBR block copolymer.

According to another characteristic of the invention, the or each blockother than a polyisoprene has a content of trans-1,4 linkages equal toor greater than 70%.

According to another characteristic of the invention, the or eachpolyisoprene block (IR) of the copolymer according to the invention hasa content of vinyl linkages (3,4 and 1,2) which is substantially between1 and 20%.

According to another characteristic of the invention, said polyisopreneblock(s) has(have) a number-average molecular weight M_(n1) of between2,500 and 20,000 g/mol, said block(s) formed of said diene elastomerhaving a number-average molecular weight M_(n2) of between 65,000 and350,000 g/mol.

It will be noted that this reduced molecular weight of the polyisopreneblock(s) makes it possible not to reduce the modulus of the blockcopolymer obtained too significantly.

Preferably, the ratio of said number-average molecular weightsM_(n1)/M_(n2) is substantially between 5 and 20%.

As regards said catalytic system which is used to prepare the blockcopolymers according to the invention, mention may be made, asrepresentative examples of said compounds A which are usable, of thefollowing organometallic compounds:

organoaluminum compounds, whether halogenated or not, such astriethylaluminum, triisobutylaluminum, diethylaluminum chloride,ethylaluminum dichloride, ethylaluminum sesquichloride or methylaluminumsesquichloride; dialkylaluminum hydrides, such as diethylaluminumhydride and diisobutylaluminum hydride.

Preferably, a trialkylaluminum the number of carbon atoms of which isfrom 1 to 12, advantageously trioctylaluminum, is used for said compoundA.

The following can particularly be used as compounds B: the hydrides H₂Baand H₂Sr, mono- or polyfunctional organic acid salts of formulae(R—COO)₂ Ba or Sr, R₁—(COO)₂ Ba or Sr, where R and R₁ are organicradicals (the first monovalent and the second divalent), thecorresponding thioacids, mono- or polyfunctional alcoholates and thecorresponding thiolates; mono- or polyfunctional phenates and thecorresponding thiophenates; Ba or Sr hydroxyacid and phenolic acid saltsand the corresponding thio-products; Ba or Sr β-diketonates such as thereaction products of Ba or Sr with acetylacetone, dibenzoylmethane,thenoyltrifluoroacetone, benzoyltrifluoroacetone or benzoylacetone;organic Ba or Sr derivatives such as those of 1,1-diphenylethylene,1,2-acenaphthylene, tetraphenylbutane, α-methylstyrene, or alternativelythose such as diphenylbarium or -strontium, bis(cyclopentadienyl)bariumor -strontium, trialkylsilylbarium or -strontium, ortriphenylsilylbarium or -strontium; mixed organic derivatives such asphenylbarium iodide, methylstrontium iodide, Ba or Sr salts of secondaryamines; ketyl metals such as Ba or Sr benzophenone, Ba or Sr cinnamoneand the corresponding alkylated products and the sulphurised homologues;radical ions of Ba and Sr such as those of naphthalene, anthracene,chrysene, diphenyl, etc.

It is also possible to use a calcium alcoholate for the compound B.

Preferably, a barium alcoholate, advantageously barium ethyl diglycolateor barium nonylphenoxide, is used for said compound B.

A non-functionalized monolithiated polyisoprene, that is to say, onewhich is devoid of any functional group such as carboxyl, amine, amide,etc., groups, and preferably obtained anionically is usable as polymerinitiator C comprising a C—Li bond.

According to a first embodiment of the invention, this catalytic systemaccording to the invention comprises a co-catalyst, resulting from theproduct of reaction in said hydrocarbon solvent of said compound A andsaid compound B, and said polymer initiator C.

According to a first example of embodiment of this first embodiment, thepreparation process according to the invention consists of implementingthe following steps:

-   -   in a first step, this co-catalyst is prepared by reacting the        two compounds A and B in said inert hydrocarbon solvent. Then        the mixture obtained is heated preferably to a temperature of        between 20 and 120° C., even more preferably between 30 and 50°        C., and for a time sufficient to permit the reaction of the two        compounds A and B (generally between 1 and 60 min., preferably        between 20 and 40 min.); then    -   in a second step, said co-catalyst is contacted with the        polymerization medium comprising said monomer(s) which is/are to        be copolymerized (which are for example in solution in a        polymerization solvent, in the case of copolymerization in        solution), with the exclusion of said polymer initiator C; then    -   in a third step, said initiator C is added to the polymerization        medium thus obtained, so as to react the mixture obtained in        said second step, then    -   in a fourth step, said functionalizing, coupling or starring        agent is added to the product of the copolymerization,

and the functionalization, coupling or starring reaction is laterstopped in order to obtain said functionalized, coupled or starred blockcopolymer.

According to a second example of embodiment of this first embodiment,the preparation process according to the invention then consists ofimplementing the following steps:

-   -   a first step which is the same as that described for said first        example; then    -   a second step which consists of adding said polymer initiator C        to the premix obtained in the first step formed by compounds A        and B, possibly after having added an alkyllithium compound to        improve the activity of the catalytic system. Preferably, this        alkyllithium compound is butyllithium; then    -   a third step which consists of adding the catalytic system thus        obtained to the polymerization medium comprising said monomer(s)        to be copolymerized (which are for example in solution in a        polymerization solvent, in the case of copolymerization in        solution); then    -   a fourth step which consists of adding said functionalizing,        coupling or starring agent to the product of the        copolymerization, and

the functionalization, coupling or starring reaction is later stopped inorder to obtain said functionalized, coupled or starred block copolymer.

The temperature conditions are the same as those of said first example.

According to a second embodiment of the invention, the catalytic systemcomprises a premix of said compounds A and C in said hydrocarbon solventand said compound B.

This second embodiment is for example carried out in the followingmanner:

-   -   in a first step, a premix of said compounds A and C in said        hydrocarbon solvent is produced, then    -   in a second step, this premix is added to the polymerization        medium comprising said monomer(s) to be copolymerized in        solution in a polymerization solvent, then    -   in a third step, said compound B is added to the mixture        obtained in said second step, then    -   in a fourth step, said functionalizing, coupling or starring        agent is added to the product of the copolymerization, and

the functionalization, coupling or starring reaction is later stopped inorder to obtain said functionalized, coupled or starred block copolymer.

In these two embodiments of the preparation process according to theinvention, a quantity of the reagents A and B is used such that themolar ratio A/B is of between 0.5 and 5, and preferably between 2.5 and4. Furthermore, a quantity of the two reagents B and C is used which issuch that the molar ratio C/B is of between 0.2 and 4, and preferablybetween 1.5 and 4.

In the case of copolymerization in solution, the polymerization solventis preferably a hydrocarbon solvent, preferably cyclohexane, and thepolymerization temperature is between 20 and 150° C., preferably between60 and 110° C.

Furthermore, the concentration of alkaline-earth metal of the catalyticsystem according to the invention is between 0.01 and 0.5 mol.l⁻¹,preferably between 0.03 and 0.25 mol.l⁻¹.

It will be noted that the copolymerization according to the inventionmay be continuous or discontinuous and that it may also be effectedwithout solvent.

With regard to the specific modifying functionalizing, coupling orstarring agents which are usable for obtaining the modified blockcopolymers according to the invention with reference to said formula(1), it will be noted that the groups R and R′ may comprise a variablenumber of carbon atoms which may for example be up to 22 (i.e. from 1 to22 carbon atoms for the alkyl groups and from 6 to 22 for thecycloalkyl, aryl or aralkyl groups).

To obtain a functionalized block copolymer according to the invention(i.e. by definition at a single chain end), a functionalizing agent isused which corresponds to said formula (1) with n=3, i.e.R₃—Sn—(O—CO—R′), the copolymer obtained being linear with two blocks(e.g. IR/SBR).

Tributyl tin acetate may be mentioned as an example of a functionalizingagent usable according to the invention.

To obtain a coupled block copolymer according to the invention, acoupling agent in accordance with formula (1) is used with n=2, i.e.R₂—Sn—(O—CO—R′)₂, this copolymer being linear with three blocks, the twoend blocks consisting of a polyisoprene (e.g. IR/SBR/IR).

Preferably, the coupling agent used is dibutyl tin diacetate.

Other coupling agents satisfying formula (1) are also usable, such asdibutyl tin dilaurate or dibutyl tin distearate, in non-limitativemanner.

To obtain a starred block copolymer according to the invention, astarring agent in accordance with formula (1) is used with n=0 or 1,i.e. Sn—(O—CO—R′)₄ or R₁—Sn—(O—CO—R′)₃, respectively for obtaining abranched copolymer with four or three branches, each branch comprisingin both these cases two blocks, of which the end block is formed of apolyisoprene (e.g. four or three branches with SBR/IR blocks).

By way of example of such starring agents which are usable according tothe invention, mention may be made of butyl tin triacetate (n=1) and tintetraacetate (n=0).

Other starring agents which correspond to said formula (1), such asethyl tin tristearate, butyl tin trioctanoate, butyl tin tristearate orbutyl tin trilaurate are also usable, in non-limitative manner.

According to one preferred characteristic of the invention, the additionof said modifying agent is carried out such that the molar ratio Sn/Li(i.e. modifying agent/lithiated polyisoprene) is between 0.2 and 1 and,even more preferably, between 0.3 and 0.8.

In fact, tests carried out in the context of the present invention haveshown that the “jump” or difference in inherent viscosity between thenon-modified block copolymer and the modified (i.e. functionalized,coupled or starred) block copolymer, which constitutes a satisfactoryindicator of the modification achieved on the copolymer, reaches anoptimum when the value of this ratio Sn/Li lies within theaforementioned ranges.

A functionalized, coupled or starred block copolymer according to theinvention, which is usable in a sulphur-cross-linkable rubbercomposition, of reduced hysteresis in the cross-linked state andcomprising carbon black as reinforcing filler, one at least of saidblocks consisting of a polyisoprene and at least one other blockconsisting of a diene elastomer other than a polyisoprene the molarratio of units originating from one or more conjugated dienes of whichis greater than 15%, is such that said block formed of a diene elastomerother than a polyisoprene is functionalized, coupled or starred by meansof an agent comprising an acetoxy group of formulaR_(n)—Sn—(O—CO—R′)_(4−n), where n is a natural integer of from 0 to 4and where R and R′ are each alkyl, cycloalkyl, aryl or aralkyl groupswhich are identical or different.

According to another characteristic of the invention, thisfunctionalized, coupled or starred block copolymer is such that the oreach block formed of a diene elastomer other than a polyisoprene has atrans-1,4 linkage content equal to or greater than 70%.

According to another characteristic of the invention, thisfunctionalized, coupled or starred block copolymer is such that thepolyisoprene block(s) has(have) a number-average molecular weight M_(n1)of between 2,500 and 20,000 g/mol, and that said block(s) formed of saiddiene elastomer has(have) a number-average molecular weight M_(n2) ofbetween 65,000 and 350,000 g/mol.

According to one example of embodiment of the invention, this blockcopolymer is functionalized by means of said functionalizing agentaccording to the invention of the formula R₃—Sn—(O—CO—R′), such thatsaid copolymer is linear with two blocks.

According to another example of embodiment of the invention, this blockcopolymer is coupled by means of said coupling agent according to theinvention of formula R₂—Sn—(O—CO—R′)₂, preferably dibutyl tin diacetate,such that said copolymer is linear with three blocks, the two end blockseach consisting of a polyisoprene.

According to another example of embodiment of the invention, this blockcopolymer is starred by means of said starring agent according to theinvention of the formula Sn—(O—CO—R′)₄ or R₁—Sn—(O—CO—R′)₃, such thatsaid copolymer is branched with four or three branches respectively,each branch comprising two blocks of which the end block is formed of apolyisoprene.

Preferably, a block copolymer according to the invention is such thatthe ratio of said number-average molecular weights M_(n1)/M_(n2) isbetween 5 and 20%.

Preferably too, a block copolymer according to the invention is suchthat the diene elastomer forming said or each block other than apolyisoprene is a copolymer of butadiene and a vinyl aromatic compound,such as styrene.

Preferably too, a block copolymer according to the invention is suchthat said or each polyisoprene block has a content of 3,4 and 1,2 vinyllinkages which is substantially between 1 and 20%.

A rubber composition according to the invention comprises a reinforcingfiller formed in its entirety or in part of carbon black and is suchthat it comprises said functionalized, coupled or starred blockcopolymer as defined previously, which makes this composition suitable,on one hand, to have improved processing ability in the non-cross-linkedstate and reduced hysteresis in the cross-linked state and, on the otherhand, to constitute a tire tread having reduced rolling resistance.

Preferably, the composition according to the invention comprises thisblock copolymer in a quantity greater than 50 phr and possibly as muchas 100 phr (phr: parts by weight per hundred parts of elastomer(s)) and,even more preferably, in a quantity of 100 phr.

Preferably, said reinforcing filler comprises in a majority proportioncarbon black (i.e. in a weight fraction greater than 50%).

Preferably too, the reinforcing filler comprises carbon black in aquantity greater than 40 phr (phr: parts by weight per hundred parts ofelastomer(s)).

As carbon black, there are suitable all the blacks which arecommercially available or conventionally used in tires, and particularlyin treads, in particular blacks of the type HAF, ISAF, SAF andpreferably blacks of series 200 or 300. As non-limitative examples ofsuch blacks, mention may be made of the blacks N234, N339, N347, N375,and also of the blacks of series 100 such as N115 and N134.

Of course, “carbon black” is also understood to mean a blend ofdifferent carbon blacks which is usable to form all or part of saidreinforcing filler.

The reinforcing filler may furthermore comprise a reinforcing inorganicfiller, preferably in a minority proportion (i.e. in a weight fractionof less than 50%).

In the present application, “reinforcing inorganic filler”, in knownmanner, is understood to mean an inorganic or mineral filler, whateverits colour and its origin (natural or synthetic), also referred to as“white” filler or sometimes “clear” filler in contrast to carbon black,this inorganic filler being capable, on its own, without any other meansthan an intermediate coupling agent, of reinforcing a rubber compositionfor tires, in other words which is capable of replacing a conventionaltire-grade carbon black filler in its reinforcement function.

Preferably, all or at the very least a majority proportion of thereinforcing inorganic filler is silica (SiO₂). The silica used may beany reinforcing silica known to the person skilled in the art, inparticular any precipitated or fumed silica having a BET surface areaand a CTAB specific surface area both of which are less than 450 m²/g,even if the highly dispersible precipitated silicas are preferred.

In the present specification, the BET specific surface area isdetermined in known manner, in accordance with the method of Brunauer,Emmett and Teller described in “The Journal of the American ChemicalSociety”, vol. 60, page 309, February 1938, and corresponding toStandard AFNOR-NFT-45007 (November 1987); the CTAB specific surface areais the external surface area determined in accordance with the sameStandard AFNOR-NFT-45007 of November 1987.

“Highly dispersible silica” is understood to mean any silica having avery substantial ability to disagglomerate and to disperse in anelastomer matrix, which can be observed in known manner by electron oroptical microscopy on thin sections. As non-limitative examples of suchpreferred highly dispersible silicas, mention may be made of the silicaPerkasil KS 430 from Akzo, the silica BV3380 from Degussa, the silicasZeosil 1165 MP and 1115 MP from Rhodia, the silica Hi-Sil 2000 from PPG,the silicas Zeopol 8741 or 8745 from Huber, and treated precipitatedsilicas such as, for example, the aluminum-“doped” silicas described inpatent specification EP-A-735088.

The physical state in which the reinforcing inorganic filler is presentis immaterial, whether it be in the form of a powder, microbeads,granules or alternatively balls. Of course, “reinforcing inorganicfiller” is also understood to mean mixtures of different reinforcinginorganic fillers, in particular of highly dispersible silicas such asdescribed above.

For example, black/silica blends or blacks partially or entirely coveredwith silica are suitable to form the reinforcing inorganic filler.

Also suitable as reinforcing inorganic fillers are carbon blacksmodified by silica, such as the fillers sold by CABOT under the name“CRX 2000”, which are described in patent specification WO-A-96/37547,aluminas (of formula Al₂O₃), such as the high-dispersibility aluminaswhich are described in European patent specification EP-A-810 258, oralternatively aluminum hydroxides, such as those described ininternational patent specification WO-A-99/28376.

The rubber composition according to the invention may furthermorecomprise, in conventional manner, a reinforcing inorganicfiller/elastomeric matrix bonding agent (also referred to as “couplingagent”), the function of which is to ensure sufficient chemical and/orphysical bonding (or coupling) between the possible reinforcinginorganic filler and the matrix, while facilitating the dispersion ofthis inorganic filler within said matrix.

In known manner, in the presence of a reinforcing inorganic filler, itis necessary to use a coupling agent the function of which is to providea sufficient chemical and/or physical connection between the inorganicfiller (surface of its particles) and the elastomer.

Such a coupling agent, which is consequently at least bifunctional, has,for example, the simplified general formula “Y-T-X”, in which:

-   -   Y represents a functional group (“Y” function) capable of        bonding physically and/or chemically with the inorganic filler,        such a bond being able to be established, for example, between a        silicon atom of the coupling agent and the surface hydroxyl        groups of the inorganic filler (e.g. surface silanols in the        case of silica),    -   X represents a functional group (“X” function) which is capable        of bonding physically and/or chemically with the diene        elastomer, i.e. via a sulphur atom,    -   T represents a divalent group making it possible to link Y and        X.

Any coupling agent likely to ensure, in the diene rubber compositionsusable for the manufacturing of tire treads, the effective bondingbetween a reinforcing inorganic filler such as silica and a dieneelastomer, in particular organosilanes or polyfunctionalpolyorganosiloxanes bearing the functions X and Y, may be used.

In particular polysulphurised silanes, which are referred to as“symmetrical” or “asymmetrical” depending on their specific structure,are used, such as those described for example in the patent documents FR2 149 339, FR 2 206 330, U.S. Pat. Nos. 3,842,111, 3,873,489, 3,978,103,3,997,581, 4,002,594, 4,072,701, 4,129,585, 5,580,919, 5,583,245,5,650,457, 5,663,358, 5,663,395, 5,663,396, 5,674,932, 5,675,014,5,684,171, 5,684,172, 5,696,197, 5,708,053, 5,892,085, EP 1 043 357, WO02/083782.

Particularly suitable for implementing the invention, without thedefinition below being limitative, are what are called “symmetrical”polysulphurised silanes which satisfy the following general formula (I):Z-A-S_(n)-A-Z, in which:  (I)

-   -   n is an integer from 2 to 8 (preferably from 2 to 5);    -   A is a divalent hydrocarbon radical (preferably C₁-C₁₈ alkylene        groups or C₆-C₁₂ arylene groups, more particularly C₁-C₁₀        alkylenes, notably C₁-C₄ alkylenes, in particular propylene);    -   Z corresponds to one of the formulae below:

-   -   in which:    -   the radicals R¹, which may or may not be substituted, and may be        identical or different, represent a C₁-C₁₈ alkyl group, a C₅-C₁₈        cycloalkyl group or a C₆-C₁₈ aryl group, (preferably C₁-C₆ alkyl        groups, cyclohexyl or phenyl, in particular C₁-C₄ alkyl groups,        more particularly methyl and/or ethyl),    -   the radicals R², which may or may not be substituted, and may be        identical or different, represent a C₁-C₁₈ alkoxyl group or a        C₅-C₁₈ cycloalkoxyl group (preferably a group selected from        among C₁-C₈ alkoxyls and C₅-C₈ cycloalkoxyls, more preferably        still a group selected from among C₁-C₄ alkoxyls, in particular        methoxyl and/or ethoxyl).

In the case of a mixture of polysulphurised alkoxysilanes in accordancewith Formula (I) above, in particular conventional, commerciallyavailable, mixtures, the average value of the “n”s is a fractionalnumber, preferably between 2 and 5, more preferably close to 4. However,the invention may also be implemented advantageously for example withdisulphurised alkoxysilanes (n=2).

As examples of polysulphurised silanes, mention will be made moreparticularly of the polysulphides (in particular disulphides,trisulphides or tetrasulphides) ofbis-((C₁-C₄)alkoxyl-(C₁-C₄)alkylsilyl-(C₁-C₄)alkyl), such as for examplebis(3-trimethoxysilylpropyl) or bis(3-triethoxysilylpropyl)polysulphides. Of these compounds, in particularbis(3-triethoxysilylpropyl) tetrasulphide, abbreviated TESPT, of theformula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, or bis(triethoxysilylpropyl) disulphide,abbreviated TESPD, of the formula [(C₂H₅O)₃Si(CH₂)₃S]₂, are used.

TESPD is sold, for example, by Degussa under the name Si75 (in the formof a mixture of disulphide—75% by weight—and of polysulphides), oralternatively by Witco under the name Silquest A1589. TESPT is sold, forexample, by Degussa under the name Si69 (or X50S when it is supported to50% by weight on carbon black), or alternatively by Osi Specialtiesunder the name Silquest A1289 (in both cases, a commercial mixture ofpolysulphides having an average value of n which is close to 4).

Mention will also be made, as examples of advantageous coupling agent,of the polysulphides (in particular disulphides, trisulphides ortetrasulphides) of bis-(mono(C₁-C₄)alkoxyl-di(C₁-C₄)alkylsilylpropyl),more particularly bis-monoethoxydimethylsilylpropyl tetrasulphide asdescribed in the aforementioned application WO 02/083782.

As examples of coupling agents other than the aforementionedpolysulphurised alkoxysilanes, mention will be made in particular of thebifunctional polyorganosiloxanes such as described in the aforementionedapplications WO99/02602 or WO01/96442, or alternatively thehydroxysilane polysulphides such as described in the aforementionedapplications WO02/30939 and WO 02/31041.

The compositions according to the invention contain, in addition to theelastomeric matrix, said reinforcing filler and possibly a reinforcinginorganic filler/elastomer(s) bonding agent, all or some of the otheradditives usually used in rubber mixes, such as plasticisers, pigments,antioxidants, antiozone waxes, a cross-linking system based on eithersulphur and/or peroxide and/or bismaleimides, cross-linkingaccelerators, extender oils, possibly one or more covering agents forthe reinforcing inorganic filler, such as alkoxysilanes, polyols,amines, etc.

A tire tread according to the invention is such that it comprises across-linkable or cross-linked rubber composition such as defined above.

A tire according to the invention is such that it comprises this tread.

The aforementioned characteristics of the present invention, as well asothers, will be better understood on reading the following descriptionof several examples of embodiment of the invention, which are given byway of illustration and not of limitation.

-   -   In the examples below, the Mooney viscosity ML(1+4) at 100° C.        is measured in accordance with Standard ASTM D 1646 of 1999,        abbreviated to ML.    -   SEC (size exclusion chromatography) was used to determine the        number-average molecular weights Mn of the copolymers obtained.        According to this technique, the macromolecules are separated        physically according to their respective sizes when swollen, in        columns filled with a porous stationary phase.

A chromatograph sold under the name “WATERS” of model “150C” is used forthe aforementioned separation. A set of two “WATERS” columns is used,the type being “STYRAGEL HT6E”.

-   -   Furthermore, carbon-13 nuclear magnetic resonance (¹³C-NMR) was        used to determine the microstructure characteristics of the        elastomers obtained. The details of this characterisation are        explained below.

The ¹³C-NMR analyses are performed using a “Bruker AM250” spectrometer.The nominal frequency of carbon 13 is 62.9 MHz. To ensure quantitativeresults, the spectra are recorded without the “nuclear Overhausereffect” (NOE). The spectral width is 240 ppm. The angle pulse used is a90° pulse the duration of which is 5 μs. Low-power decoupling with awide proton band are used to eliminate scalar ¹H—¹³C coupling during ¹³Cacquisition. The sequence repetition time is 4 seconds. The number oftransients accumulated to increase the signal/noise ratio is 8192. Thespectra are calibrated against the CDCl₃ band at 77 ppm.

-   -   The assay technique known as “near-infrared” (NIR), an indirect        method using “control” elastomers the microstructure of which        was measured by the ¹³C-NMR technique, was also used. The        quantitative relationship (Beer-Lambert law) prevailing between        the distribution of the monomers in an elastomer and the shape        of the elastomer's NIR spectrum is exploited. This technique is        carried out in two stages:

a) Calibration:

-   -   The respective spectra of the “control” elastomers are acquired.    -   A mathematical model is constructed which associates a        microstructure to a given spectrum using the PLS (partial least        squares) regression method, which is based on a factorial        analysis of the spectral data. The following two documents        provide a thorough description of the theory and practice of        this “multi-variant” method of data analysis:

(1) P. GELADI and B. R. KOWALSKI

“Partial Least Squares regression: a tutorial”,

Analytica Chimica Acta, vol. 185, 1-17 (1986).

(2) M. TENENHAUS

“La régression PLS—Théorie et pratique” Paris, Editions Technip (1998).

2) Measurement:

-   -   The spectrum of the sample is recorded.    -   The microstructure is calculated.

EXAMPLES Coupled Copolymer A According to the Invention with ThreeBlocks IR/SBR/IR, Compared with Coupled “Control” Copolymers SBR B, C,D, E

1) Preparation of a Coupled Copolymer A with Three Blocks IR/SBR/IRAccording to the Invention

Preparation of a Co-catalyst Included in a Catalytic System According tothe Invention:

20 ml of cyclohexane, 3.8×10⁻³ mol of barium ethyl diglycolate(component B in solution in cyclohexane) and 13.3×10⁻³ mol oftrioctylaluminum (component A in solution in cyclohexane) are introducedinto a 0.25 l bottle which is kept under nitrogen. This mixture isstirred for 20 minutes at 40° C., and forms said co-catalyst.

Preparation of the Polymer Initiator C Formed of a LithiatedPolyisoprene:

154 ml of cyclohexane and 44 ml (30 g) of isoprene are introduced into a0.25 l bottle which is kept under nitrogen. 3×10⁻³ mol of s-BuLi isadded and the polymerization is effected at 50° C. for 45 minutes. Thelithiated polyisoprene of Mn=10,000 g/mol thus obtained is kept undernitrogen in a freezer at −20° C.

Copolymerization Using the Catalytic System According to the Invention:

Cyclohexane (154 ml), butadiene and styrene are introduced into a 0.25 l“control” bottle kept under nitrogen and into another, identical, bottleintended for implementing the invention, in the respective weight ratios1/ 0.113/ 0.102. 0.62 ml of said co-catalyst (or 62.5 μmol of bariumequivalent), then 147 μmol of said lithiated polyisoprene is added. Thepolymerization is carried out in each of the two bottles at 80° C., andthe amount of monomer converted is 68% after 155 min. This amount isdetermined by weighing an extract dried at 110° C., at the reducedpressure of 200 mmHg.

In the “control” bottle containing the polymerization product, thelatter is stopped with an excess of methanol relative to the lithium.The inherent viscosity measured (“initial” viscosity) is 1.39 dl/g.

Coupling Using a Coupling Agent According to the Invention:

A solution of dibutyl tin diacetate (59 μmol) is injected into the otherbottle containing this same polymerization product. The ratio Sn/Li istherefore 0.40. After 15 minutes' reaction at 80° C., the couplingreaction is stopped with an excess of methanol relative to the lithium.The “final” inherent viscosity measured is 1.81 dl/g.

The viscosity jump, defined as the ratio of said “final” viscosity tosaid “initial” viscosity, here is 1.30. The viscosity ML of the polymerthus coupled is 55.

The copolymer A with three blocks IR/SBR/IR thus obtained is subjectedto antioxidant treatment by addition of 0.35 parts per hundred parts ofelastomers (phr) of 4,4′-methylene-bis-2,6-tert-butylphenol and 0.10parts per hundred parts of elastomers (phr) ofN-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine. This copolymer isrecovered by drying in an oven at 60° C. in a stream of nitrogen.

The molecular weight Mn of this copolymer A, determined by the SECtechnique, is 117,000 g/mol. The amount of residual free polyisopreneestimated by SEC is less than 2%.

The microstructure of this copolymer A is determined by ¹³C NMR:

the weight content of BR trans-1,4 is 79%, that of BR cis-1,4 is 16% andthat of BR 1,2 is 5% (each of these three amounts relates to butadieneunits);

the weight content of styrene is 28%;

the weight content of IR 3,4 units is 8%, that of the IR trans-1,4 unitsis 24% and that of the IR cis-1,4 units is 68% (these three amountsrelate to the polyisoprene block).

Finally, the weight fraction of this IR block in this copolymer A is 8%.

2) Preparation of a “Control” SBR B Coupled Via Dibutyl Tin Diacetate:

This “control” SBR B is prepared by copolymerization of styrene andbutadiene carried out in accordance with § 1) above, except for the factthat the copolymerization is initiated by means of n-butyllithium aslithiated initiator, instead of the aforementioned lithiatedpolyisoprene (the co-catalyst used being the same as previously).

Cyclohexane (154 ml), butadiene and styrene are introduced into a 0.25 l“control” bottle kept under nitrogen and into another, identical, bottleintended for implementing the coupling, in the respective weight ratios1/ 0.108/ 0.100. 1.5 ml of said co-catalyst (or 150 μmol of bariumequivalent), then 412 μmol of n-butyllithium is added. Thepolymerization is carried out in each of these two bottles at 80° C.,and the amount of monomer converted is 59% after 18 min. This amount isdetermined by weighing an extract dried at 110° C., at the reducedpressure of 200 mmHg.

In the “control” bottle containing the product of the copolymerization,the latter is stopped with an excess of methanol relative to thelithium. The inherent viscosity measured (“initial” viscosity) is 1.07dl/g.

A solution of dibutyl tin diacetate (125 μmol) is injected into theother bottle containing this same polymerization product. The ratioSn/Li is therefore 0.30. After 15 minutes' reaction at 80° C., thecoupling reaction is stopped with an excess of methanol relative to thelithium. The “final” inherent viscosity measured is 1.60 dl/g.

The viscosity jump (“final” viscosity/“initial” viscosity) here is 1.50.

The viscosity ML of the copolymer thus coupled is 51.

The SBR B obtained is subjected to the aforementioned antioxidanttreatment and to the same drying operations as in § 1) above.

The molecular weight Mn of the copolymer B SBR obtained, determined bySEC, is 108,000 g/mol.

The microstructure of this “control” copolymer B is determined bynear-infrared spectroscopy (NIR):

the weight content of BR trans-1,4 is 85%, that of BR cis-1,4 is 12% andthat of BR 1,2 is 3% (each of these three amounts relates to butadieneunits).

The weight content of styrene is 30%.

3) Preparation of a “Control” SBR C Coupled Via Dichlorodibutyl Tin:

This “control” SBR C is prepared by copolymerization of styrene andbutadiene carried out in accordance with section § 2) above, except forthe fact that the coupling agent used is dichlorodibutyl tin, i.e. notin accordance with the present invention (the co-catalyst used being thesame as previously).

Cyclohexane (154 ml), butadiene and styrene are introduced into a 0.25 lbottle kept under nitrogen and into another, identical, bottle intendedfor implementing the coupling, in the respective weight ratios 1/ 0.108/0.100. 1 ml of said co-catalyst (or 100 μmol of barium equivalent), then275 μmol of n-butyllithium is added. The polymerization is carried outin each of these two bottles at 80° C., and the amount of monomerconverted is 53% after 15 min. This amount is determined by weighing anextract dried at 110° C., at the reduced pressure of 200 mmHg.

In the “control” bottle containing the product of the copolymerization,the latter is stopped with an excess of methanol relative to thelithium. The inherent viscosity measured (“initial” viscosity) is 1.28dl/g.

A solution of dichlorodibutyl tin (200 μmol) is injected into the otherbottle containing the same copolymerization product. The ratio Sn/Li istherefore 0.73. After 15 minutes' reaction at 80° C., the couplingreaction is stopped with an excess of methanol relative to the lithium.The inherent viscosity measured (“final” viscosity) is 1.49 dl/g.

The viscosity jump (“final” viscosity/“initial” viscosity), is only 1.16(in comparison with the viscosity jump of 1.50 obtained in § 2) for theSBR B coupled with a coupling agent according to the inventionconsisting of dibutyl tin diacetate).

The SBR C obtained is subjected to the aforementioned antioxidanttreatment and to the same drying operations.

The microstructure of this “control” SBR C is determined bynear-infrared spectroscopy (NIR):

the weight content of BR trans-1,4 is 85%, that of BR cis-1,4 is 12% andthat of BR 1,2 is 3% (each of these three amounts relates to butadieneunits).

The weight content of styrene is 30%.

4) Preparation of a “Control” SBR D Coupled Via Dichlorodimethyl Tin:

This “control” SBR D is prepared by copolymerization of styrene andbutadiene carried out in accordance with § 2) above, except for the factthat the coupling agent used is dichlorodimethyl tin, i.e. not inaccordance with the invention (the co-catalyst used being the same aspreviously).

Cyclohexane (154 ml), butadiene and styrene are introduced into a 0.25 lbottle kept under nitrogen and into another, identical, bottle intendedfor implementing the coupling, in the respective weight ratios 1/ 0.108/0.100. 1 ml of said co-catalyst (or 100 μmol of barium equivalent), then275 μmol of n-butyllithium is added. The polymerization is carried outin each of these two bottles at 80° C., and the amount of monomerconverted is 68% after 30 min. This amount is determined by weighing anextract dried at 110° C., at the reduced pressure of 200 mmHg.

In the “control” bottle containing the product of the copolymerization,the latter is stopped with an excess of methanol relative to thelithium. The inherent viscosity measured (“initial” viscosity) is 1.29dl/g.

A solution of dichlorodimethyl tin (200 μmol) is injected into the otherbottle containing the same copolymerization product. The ratio Sn/Li istherefore 0.73. After 15 minutes' reaction at 80° C., the couplingreaction is stopped with an excess of methanol relative to the lithium.The inherent viscosity measured (“final” viscosity) is 1.44 dl/g.

The viscosity jump (“final” viscosity/“initial” viscosity) is only 1.12(in comparison with the viscosity jump of 1.50 obtained in § 2) for theSBR B coupled with a coupling agent according to the inventionconsisting of dibutyl tin diacetate).

The SBR D obtained is subjected to the aforementioned antioxidanttreatment and to the same drying operations.

The microstructure of this “control” copolymer D is determined bynear-infrared spectroscopy:

the weight content of BR trans-1,4 is 85%, that of BR cis-1,4 is 12% andthat of BR 1,2 is 3% (each of these three amounts relates to butadieneunits).

The weight content of styrene is 30%.

5) Preparation of a “Control” SBR E Coupled Via Dichlorodiphenyl Tin:

This “control” SBR E is prepared by copolymerization of styrene andbutadiene carried out in accordance with § 2) above, except for the factthat the coupling agent used is dichlorodiphenyl tin, i.e. which is notin accordance with the invention (the co-catalyst used being the same aspreviously).

Cyclohexane (154 ml), butadiene and styrene are introduced into a 0.25 l“control” bottle kept under nitrogen and into another, identical, bottleintended for implementing the coupling, in the respective weight ratios1/ 0.108/ 0.100. 1 ml of said co-catalyst (or 100 μmol of bariumequivalent), then 275 μmol of n-butyllithium is added. Thepolymerization is carried out in each of these two bottles at 80° C.,and the amount of monomer converted is 60% after 28 min. This amount isdetermined by weighing an extract dried at 110° C., at the reducedpressure of 200 mmHg.

In the “control” bottle containing the product of the copolymerization,the latter is stopped with an excess of methanol relative to lithium.The inherent viscosity measured (“initial” viscosity) is 1.33 dl/g.

A solution of dichlorodiphenyl tin (200 μmol) is injected into the otherbottle containing this same polymerization product. The ratio Sn/Li istherefore 0.73. After 15 minutes' reaction at 80° C., the couplingreaction is stopped with an excess of methanol relative to the lithium.The inherent viscosity measured (“final” viscosity) is 1.50 dl/g.

The viscosity jump (“final” viscosity/“initial” viscosity) is here 1.13(in comparison with the viscosity jump of 1.50 obtained in § 2) for theSBR B coupled with a coupling agent according to the inventionconsisting of dibutyl tin diacetate).

The SBR E obtained is subjected to the aforementioned antioxidanttreatment and to the same drying operations.

The microstructure of this “control” copolymer E is determined bynear-infrared spectroscopy:

the weight content of BR trans-1,4 is 85%, that of BR cis-1,4 is 12% andthat of BR 1,2 is 3% (each of these three amounts relates to butadieneunits).

The weight content of styrene is 30%.

1. A process for the preparation of a functionalized, coupled or starredblock copolymer which is usable in a sulphur-cross-linkable rubbercomposition comprising carbon black and of reduced hysteresis in thecross-linked state, one at least of said blocks consisting of apolyisoprene and at least one other block consisting of a dieneelastomer other than a polyisoprene the molar ratio of units originatingfrom one or more conjugated dienes of which is greater than 15%, whereinsaid process comprises: (i) copolymerization of one or more monomerscomprising at least one conjugated diene other than isoprene using acatalytic system comprising a hydrocarbon solvent, a compound A of ametal of group IIIA, a compound B of an alkaline-earth metal and apolymer initiator C comprising a C—Li bond which is formed of amonolithiated non-functionalized polyisoprene intended to form saidpolyisoprene block, and (ii) addition to the product of saidcopolymerization of a functionalizing, coupling or starring agentcomprising an acetoxy group of the formula R_(n)—Sn—(O—CO—R′)_(4−n),where n is a natural integer of from 0 to 4 and where R and R′ are eachalkyl, cycloalkyl, aryl or aralkyl groups which are identical ordifferent, so that said block formed of a diene elastomer other than apolyisoprene is functionalized, coupled or starred.
 2. The process forthe preparation of a functionalized, coupled or starred block copolymeraccording to claim 1, wherein said each block formed of a dieneelastomer other than a polyisoprene has a trans-1,4 linkage contentequal to or greater than 70%.
 3. The process for the preparation of afunctionalized, coupled or starred block copolymer according to claim 1,wherein said polyisoprene block(s) has(have) a number-average molecularweight M_(n1) of between 2,500 and 20,000 g/mol, said block(s) formed ofsaid diene elastomer having a number-average molecular weight M_(n2) ofbetween 65,000 and 350,000 g/mol.
 4. The process for the preparation ofa functionalized block copolymer according to claim 1, wherein theaddition of said functionalizing agent which satisfies the formulaR₃—Sn—(O—CO—R′) with n=3, forms a linear copolymer with two blocks. 5.The process for the preparation of a coupled block copolymer accordingto claim 1, wherein the addition of said coupling agent which satisfiesthe formula R₂—Sn—(O—CO—R′)₂ with n=2, forms a linear copolymer withthree blocks, the two end blocks of which each consisting of apolyisoprene.
 6. The process for the preparation of a coupled blockcopolymer according to claim 5, wherein said coupling agent is dibutyltin diacetate.
 7. The process for the preparation of a starred blockcopolymer according to claim 1, wherein the addition of said starringagent which satisfies the formula Sn—(O—CO—R′)₄ or R—Sn—(O—CO—R′)₃ withn=0 or 1, respectively forms a branched copolymer with four or threebranches, each branch comprising two blocks of which the end block isformed of a polyisoprene.
 8. The process for the preparation of afunctionalized, coupled or starred block copolymer according to claim 1,wherein the addition of said functionalizing, coupling or starring agentis carried out such that the molar ratio Sn/Li is between 0.2 and
 1. 9.The process for the preparation of a functionalized, coupled or starredblock copolymer according to claim 1, wherein the ratio of saidnumber-average molecular weights M_(n1)/M_(n2) is between 5 and 20%. 10.The process for the preparation of a functionalized, coupled or starredblock copolymer according to claim 1, wherein said catalytic systemcomprises a co-catalyst, resulting from the product of reaction in saidhydrocarbon solvent of said compound A and said compound B, and saidpolymer initiator C.
 11. The process for the preparation of afunctionalized, coupled or starred block copolymer according to claim10, wherein it comprises: in a first step, preparing said co-catalyst byreacting said metal compounds A and B with each other in saidhydrocarbon solvent, in a second step, contacting said co-catalyst withthe polymerization medium comprising said monomer(s) which is/are to becopolymerized in solution in a polymerization solvent, with theexclusion of said polymer initiator C, in a third step, reacting themixture obtained in said second step by means of said polymer initiatorC, in a fourth step, adding said functionalizing, coupling or starringagent to the product of the copolymerization, and stopping thefunctionalization, coupling or starring reaction in order to obtain saidfunctionalized, coupled or starred block copolymer.
 12. The process forthe preparation of a functionalized, coupled or starred block copolymeraccording to claim 10, wherein it comprises: in a first step, preparingsaid co-catalyst by reacting said metal compounds A and B with eachother in said hydrocarbon solvent, in a second step, adding said polymerinitiator C to the co-catalyst obtained in the first step, in a thirdstep, adding the catalytic system thus obtained to the polymerizationmedium comprising said monomer(s) to be copolymerized in solution in apolymerization solvent, in a fourth step, adding said functionalizing,coupling or starring agent to the product of the copolymerization, andstopping the functionalization, coupling or starring reaction in orderto obtain said functionalized, coupled or starred block copolymer. 13.The process for the preparation of a functionalized, coupled or starredblock copolymer according to claim 1, wherein it comprises: in a firststep, preparing a premix of said compounds A and C in said hydrocarbonsolvent, then in a second step, adding this premix to the polymerizationmedium comprising said monomer(s) to be copolymerized in solution in apolymerization solvent, in a third step, adding said compound B to themixture obtained in said second step, in a fourth step, adding saidfunctionalizing, coupling or starring agent to the product of thecopolymerization, and stopping the functionalization, coupling orstarring reaction in order to obtain said functionalized, coupled orstarred block copolymer.
 14. The process for the preparation of afunctionalized, coupled or starred block copolymer according to claim 1,wherein said compound A is a trialkylaluminum, the number of carbonatoms of which varies from 1 to
 12. 15. The process for the preparationof a functionalized, coupled or starred block copolymer according toclaim 1, wherein said compound B is an alcoholate of barium, strontiumor calcium.
 16. The process for the preparation of a functionalized,coupled or starred block copolymer according to claim 11, wherein saidfirst step comprises the production of a premix of said compounds A andB in said hydrocarbon solvent such that the molar ratio A/B is between0.5 and 5, then heating said premix to a temperature of between 20° C.and 120° C.
 17. The process for the preparation of a functionalized,coupled or starred block copolymer according to claim 1, wherein saidhydrocarbon solvent comprises toluene and/or cyclohexane.
 18. Theprocess for the preparation of a functionalized, coupled or starredblock copolymer according to claim 1, wherein the molar ratio compoundC/compound B is between 0.2 and
 4. 19. The process for the preparationof a functionalized, coupled or starred block copolymer according toclaim 1, wherein said diene elastomer forming said or each block otherthan a polyisoprene is a copolymer of butadiene and a vinyl aromaticcompound.
 20. A functionalized, coupled or starred block copolymer whichis usable in a sulphur-cross-linkable rubber composition comprisingcarbon black as reinforcing filler and of reduced hysteresis in thecross-linked state, one at least of said blocks consisting of apolyisoprene and at least one other block consisting of a dieneelastomer other than a polyisoprene, the molar ratio of unitsoriginating from one or more conjugated dienes of which is greater than15%, wherein said block formed of a diene elastomer other than apolyisoprene is functionalized, coupled or starred by means of an agentcomprising an acetoxy group of formula R_(n)—Sn—(O—CO—R′)_(4−n), where nis a natural integer of from 0 to 4 and where R and R′ are each alkyl,cycloalkyl, aryl or aralkyl groups which are identical or different. 21.The functionalized, coupled or starred block copolymer according toclaim 20, wherein said or each block formed of a diene elastomer otherthan a polyisoprene has a trans-1,4 linkage content equal to or greaterthan 70%.
 22. The functionalized, coupled or starred block copolymeraccording to claim 20, wherein said polyisoprene block(s) has(have) anumber-average molecular weight M_(n1) of between 2,500 and 20,000g/mol, and in that said block(s) formed of said diene elastomer has/havea number-average molecular weight M_(n2) of between 65,000 and 350,000g/mol.
 23. The functionalized block copolymer according to claim 20,wherein said functionalizing agent satisfies the formulaR₃—Sn—(O—CO—R′), such that said copolymer is linear with two blocks. 24.The coupled block copolymer according to claim 20, wherein said couplingagent is of formula R₂—Sn—(O—CO—R′)₂, such that said copolymer is linearwith three blocks, the two end blocks each consisting of a polyisoprene.25. The coupled block copolymer according to claim 24, wherein saidcoupling agent is dibutyl tin diacetate.
 26. The starred block copolymeraccording to claim 20, wherein said starring agent is of the formulaSn—(O—CO—R′)₄ or R₁—Sn—(O—CO—R′)₃, respectively to obtain a branchedcopolymer with four or three branches, each branch comprising two blocksof which the end block is formed of a polyisoprene.
 27. Thefunctionalized, coupled or starred block copolymer according to claim20, wherein the ratio of said number-average molecular weightsM_(n1)/M_(n2) is between 5 and 20%.
 28. The functionalized, coupled orstarred block copolymer according to claim 20, wherein said dieneelastomer forming said or each block other than a polyisoprene is acopolymer of butadiene and a vinyl aromatic compound.
 29. Thefunctionalized, coupled or starred block copolymer according to claim20, wherein each polyisoprene block has a content of 3,4 and 1,2 vinyllinkages which is between about 1 and 20%.
 30. A cross-linkable orcross-linked rubber composition, having a reduced hysteresis in thecross-linked state and usable to form a tire tread, said compositioncomprising a reinforcing filler formed at least in part of carbon black,wherein it comprises a functionalized, coupled or starred blockcopolymer according to claim
 20. 31. The rubber composition according toclaim 30, wherein it comprises said block copolymer in a quantitygreater than 50 phr and as much as 100 phr.
 32. The rubber compositionaccording to claim 31, wherein it comprises said block copolymer in aquantity of 100 phr.
 33. The rubber composition according to claim 30,wherein said reinforcing filler comprises carbon black in a majorityproportion.
 34. The rubber composition according to claim 30, whereinsaid reinforcing filler comprises carbon black in a quantity greaterthan 40 phr.
 35. A tire tread usable for reducing the rolling resistanceof a tire incorporating same, wherein it comprises a rubber compositionaccording to claim
 30. 36. A tire having reduced rolling resistance,wherein it comprises a tread according to claim 35.